The objective of the proposed research is to increase our understanding of the regulation of gene expression in human cells through the study of spontaneous and induced mutants of diploid fibroblasts and lymphoblasts in culture. Three classes of potential regulatory mutants will be sought in the arginine and cysteine biosynthetic pathways. These include mutants that have "turned on" or "induced" a gene not normally expressed in that cell type, mutants that have quantitatively altered levels of gene expression and mutants that have regained a gene function not expressed in a particular naturally occurring or experimentally induced variant cell line. Mutants will be induced with a variety of chemical and physical agents and selected in arginine- or cystine- deficient medium supplemented with an arginine or cysteine precursor and in some cases with a toxic amino acid analogue such as canavanine. They will be analyzed biochemically and genetically to determine the enzyme activities affected, the primary alterations in the cell genomes, and the chromosomal location and dominance relationships of these new mutations. Analysis of these cells with altered control mechanisms should enhance our understanding of growth, both normal as in differentiation and abnormal as in cancer. At the same time we plan to characterize in further detail normal cellular control mechanisms in diploid human cells, in particular those affecting the expression of the urea cycle enzymes argininosuccinate synthetase and argininosuccinase. These mechanisms will be constrasted with those in heteroploid human cells to determine whether the chromosomally abnormal, neeplastic cells exhibit a generalized derepression of genetic information.